The present invention relates to rotating electrical machines, and in particular to rotating electrical machines having permanent magnets.
Rotating electrical machines, such as motors and generators, generally comprise a rotor and a stator, which are arranged such that a magnetic flux is developed between the two. In a permanent magnet (PM) type machine, a number of permanent magnets are usually mounted on the rotor, while the stator is provided with stator windings. The permanent magnets cause a magnetic flux to flow across the air gap between the rotor and the stator. In the case of a generator, when the rotor is rotated by a prime mover, the rotating magnetic field causes an electrical current to flow in the stator windings, thereby generating the output power. In the case of a motor, an electrical current is supplied to the stator windings and the thus generated magnetic field causes the rotor to rotate.
Permanent magnet type machines have many advantages, including high power density, high efficiency, compact size and ease of manufacture. A significant disadvantage is the lack of field control within the machine. This can create problems when operating the machine as a generator or motor. When PM machines are operated as generators the output voltage varies with load current and cannot be kept constant. This poor voltage regulation is unacceptable for some load types, limiting the application of PM machines. When PM machines are operated as a motor the emf (electromotive force) generated within the motor increases with speed. The supply voltage to the motor is required to be greater than this internally generated emf, increasing converter costs. A popular strategy for minimising converter costs is to reduce the internally generated emf by suppressing the field within the machine by orientating the armature field produced by the armature current. This is known as ‘field weakening’ control but suffers reduced efficiency as the demagnetising current is supplied from the armature. Furthermore for applications that require both a combination of motor/generator operation such as hybrid vehicles the ability to control the field is increasingly important. For example overload conditions can be accommodated by increasing the field within the machine rather than increasing armature current thus minimising converter costs. In addition system efficiency improvements may be made as the flexibility of field control can minimise losses within the machine and converter for different operating speeds and torques.
WO 03/003546, the contents of which are incorporated herein by reference, discloses an axial flux machine with a rotor and an annular stator. The rotor has two discs mounted on a common rotor shaft, one on either side of the stator which carries field windings. Each rotor disc has two permanent magnets diametrically opposite one another on its face adjacent the stator and two pole pieces of non-magnetised ferromagnetic material diametrically opposite one another on the same face of the rotor disc. A control winding is carried by the stator in its central aperture. The control winding can be energized to establish a control field which establishes a closed loop of magnetic flux through each juxtaposed magnet and non-magnetised pole piece and thereby opposes armature reaction.